1. Field of the Invention
The present invention relates to a wiring board suitable for providing semiconductor chip mounting boards, motherboards, substrates for probe cards, and so on.
2. Description of the Related Art
Recent electronic products have been improved to meet the requirement for higher performance and further size reduction. Accordingly, high density mounting techniques are developed for electronic parts to be incorporated in those small electronic products. To attain such high density mounting, semiconductor chips are often designed as bare chips that are surface-mountable on the wiring board (flip-chip mounting). For mounting semiconductor chips with high density, use is often made of multilayer wiring boards, which are suitable for multi-pin type chips.
These semiconductor chips, mounted on a multilayer wiring board, are often packaged to provide a semiconductor unit. Such a unit or package may be mounted on a motherboard, serving as an integral part of the desired electronic circuit. The motherboard used for such purposes may also have a multilayer wiring structure for achieving high density wiring. A multilayer wiring structure is also employed for making probe cards used for measurement or inspection of electronic products such as individual semiconductor chips or semiconductor wafers with a number of semiconductor elements built in.
In the flip-chip mounting, an under filler is generally used to fill a gap between the wiring board and the semiconductor chips mounted thereon. If the under filler is not used, electrical connection between the wiring board and the semiconductor chips is often not reliable enough because of the difference in thermal expansion coefficient between the wiring board and the semiconductor chips. Typically, a semiconductor chip made of a common material has a thermal expansion coefficient of about 3.5 ppm/° C. as viewed in the surface-spreading direction (which is perpendicular to the normal of the principal plane of the chip). On the other hand, a typical wiring board including a core substrate provided by a glass epoxy substrate has a thermal expansion coefficient of about 12˜20 ppm/° C. in the surface-spreading direction. Thus, the difference in the thermal expansion coefficient between the two is relatively large. Due to this, any change in surrounding temperature can produce stress in the connecting portions between the wiring board and the semiconductor chips. When the stress at the electrical connection exceeds a limit, the boundary surface between bumps of the semiconductor chip and the electrode pads of the wiring board can easily crack or be separated from each other. The under filler applied between the semiconductor chip and the wiring board serves to mitigate the stress at the connecting portions.
However, when a large semiconductor chip is mounted on a wiring board, the stress reduction by the under filler alone is often insufficient to ensure desirable reliability. This is because the difference in thermal expansion between the semiconductor chip and the wiring board increases as the chip becomes larger, whereby an unduly great stress can be produced at the connecting portions between the chip and the wiring board. The same problem can occur in a case where a large semiconductor wafer or chip is mounted on a probe card.
The above problem resulting from the difference in the thermal expansion coefficient can be eliminated or reduced by using a wiring board of a small thermal expansion coefficient. Such a wiring board may include a core substrate made of a metal having a small thermal expansion coefficient. Examples of the metal for a core substrate may be aluminum, copper, silicon steel, nickel-iron alloy, or CIC (a clad having copper/Invar/copper layers). Japanese patent application laid-open No. 11(1999)-112145 and No. 2000-138453 disclose a wiring board provided with a metal core substrate. However, since a metal material has a considerably great specific gravity, the resultant wiring board is disadvantageously heavy. In addition, it is rather difficult to perform fine processing (boring, thin plate working, etc.) with respect to a metal core substrate.
It is conventionally known that the thermal expansion of a wiring board can also be reduced by using a carbon fiber material. Typically the thermal expansion coefficient of a carbon fiber is about −5˜3 ppm/° C. In this connection, Japanese patent application laid-open No. 60(1985)-140898 discloses a wiring board having a multilayer structure in which insulating layers (containing carbon fiber sheets) and copper wiring layers are alternately stacked. Japanese patent application laid-open No. 11(1999)-40902 discloses a multilayer wiring board including a core substrate which contains a carbon fiber sheet. On each side of the core substrate, an insulating layer (prepreg containing glass fiber) and a copper wiring layer are stacked. Japanese patent application laid-open No. 2001-332828 discloses a multilayer wiring board including a core substrate which contains a carbon fiber sheet. On each side of the core substrate, an insulating layer (a prepreg containing no glass fiber) and a copper wiring layer are stacked. Since carbon fiber undergoes small thermal expansion, the insulating layer and the core substrate have a small expansion coefficient. Accordingly, the wiring board, including such an insulating layer and a core substrate, can have a small expansion coefficient in the surface-spreading direction.
While having the above advantage, the conventional wiring boards may suffer the following drawbacks.
In the conventional wiring board, as noted above, the incorporated carbon fiber sheet prevents the core substrate (or the insulating layer) from expanding in the surface-spreading direction. However, as viewed in the thickness direction (perpendicular to the surface-spreading direction), the thermal expansion coefficient of the core substrate is rendered greater than when no carbon fiber sheet is contained. When the expansion coefficient of the core substrate in the surface-spreading direction is below 10 ppm/° C., for instance, the core substrate will expand greatly in the thickness direction of the wiring board.
The reason why such a phenomenon can happen is as follows. In general, a resin material has a relatively great thermal expansion coefficient. According to the prior art arrangement of the core substrate, however, the resin expansion in the surface-spreading direction is strictly restricted by the integrated carbon fiber sheet. As a reaction of this, the resin material tends to expand greatly in the thickness direction of the board, in which the carbon fiber sheet can exert no restriction. Unfavorably, such expansion can break a through-hole via which may extend in the thickness direction of the wiring board through the core substrate.